The invention relates to an electron-beam heated vapor source assembly for use in the field of vaporizing materials in high vacuum and, especially the field of deposition coatings on substrates.
Electron-beam heated vapor sources are frequently used in vacuum coating systems. One such vapor source is described in U.S. Pat. No. 3,710,072, issued to Robert L. Schrader and Kazumi N. Tsujimoto, and assigned to the assignee of the present application. This vapor source comprised a single crucible, a single electron-beam gun, and a magnetic means for deflecting the electron beam through an arcuate path from the electron-beam gun to the crucible. The magnetic deflecting means included two large, parallel plates of magnetic material which were positioned on opposite sides of the electron-beam gun and extended on opposite sides of the crucible The plates were polarized by interconnecting them with a single permanent magnet near their ends which were further from the electron-beam gun. The electron beam was deflected by the magnetic field produced between the pole pieces. Three electromagnetic coils were arranged in a generally U-shaped structure to enable the beam to be swept across the crucible The crucible of this prior vapor source was a copper block having a frustroconical wall for containing the material to be vaporized The wall diverged upwardly at an angle of about 15.degree. from the vertical. A second frustroconical section extends from the upper rim of the material containing wall and formed an upper cone for directing the vapor flow, and protecting other parts of the source from condensation of vapor. This upper section diverged at a larger angle, typically about 45.degree., and provided a hopper for melting loosely compacted material. The specification of U.S. Pat. No. 3,710,072 is incorporated herein by reference.
Electron-beam heated vapor sources having a plurality of crucibles for vaporizing different materials are also known. One such source is the model STIH-270-1 four-hearth source sold by the Edwards High Vacuum International Division of the assignee of the present application. This source has a hearth assembly containing four separate compartments for material to be vaporized and a means for rotating the assembly about an axis so that each compartment may be placed into the path of the electron beam.
When vaporizing some materials, such as aluminum and aluminum alloys in a copper or copper-alloy crucible, a problem sometimes occurs after material has been added to replenish the crucible. The ratio of vaporization rate to electron-beam power may decrease for successive replenishments. This is believed to occur because there may exist regions where the meniscus of the molten pool encounters previously condensed material on the wall of the crucible. This phenomenon is termed a "thermal short" since a portion of the beam power is diverted from vaporization into the crucible cooling water. In extreme cases, sufficient power may be diverted to a particular spot on the wall that the crucible wall disintegrates with resulting loss of molten material from the pool.